Polycarbonate resins have high impact resistance with ductility to notch or crack propagation at an average of up to about 0.2 inches thickness when the incident notch is 10 mils (thousandths) of an inch in radius. Above this average thickness the impact resistance and ductility of polycarbonate resins decline. This phenomena is commonly found in glassy plastics and is referred to as the critical thickness for notched impact resistance of a glassy plastic.
Additionally, the impact strength of notched polycarbonate resins decreases as temperatures decrease below about -5.degree. C. and also after aging the polymers at elevated temperatures above about 100.degree. C. These temperatures are commonly found in applications where extremes of heat and cold are to be expected.
Thus, it is desirable to use a composition which extends the impact strength and ductility of polycarbonate resins to variable thickness parts or articles of use which resist embrittlement upon exposure to high or low temperatures in a notched or scratched condition.
Compositions are known which extend the high impact resistance and ductility characteristics of polycarbonate resins to parts beyond the critical thickness and under low and high temperature aging conditions, but many of these compositions suffer from incompatabilities of the polymeric components which results in poor weld-line and knit line strengths of fabricated parts as evidenced by low double-gate impact strengths when measured according to ASTM D256.
It has been found that a composition of a polycarbonate resin, a linear low density polyolefin and a multiphase composite interpolymer of an acrylate and a methacrylate has superior impact properties at temperatures as low as about -30.degree. C. In addition, the compositions of this invention also have good weld line strengths as measured in accordance with ASTM D256, lowered melt viscosities and improved heat stability as compared to unmodified polycarbonate compositions. Furthermore, the compositions of the invention have a wider shear sensitivity, improved hydrolytic stability and enhanced chemical resistance.
Compositions according to the present invention have improved reprocessability which is of significant importance in reducing or eliminating losses due to scrap materials that cannot be re-extruded without loss of properties.
It has also been found that the low temperature impact strength of a composition of a polycarbonate, a multiphase interpolymer and a linear low density polyethylene is affected by the amount of the multiphase interpolymer and the amount of the linear low density polyethylene that are present in the composition. When levels of the linear low density polyethylene are below about 6.0 parts per hundred parts by weight of total composition (phr) it is desirable to employ about 4.0 phr of the multiphase composite interpolymer to achieve fully ductile low temperature impact strengths because when less than about 4.0 phr of the multiphase composite interpolymer are employed, with less than about 6.0 phr of the linear low density polyethylene, the test samples are brittle. If about 4.0 phr of the multiphase composite interpolymer is present in the composition, then the amount of linear low density polyethylene may be reduced to about 2.0 phr without adversely affecting the ductile impact strength.
It is to be understood that the invention is not limited to the polycarbonate compositions which have the optimum ductile impact strengths which are achieved by selection of the minimum effective amounts of the multiphase composite interpolymer and the linear low density polyethylene.
The especially preferred compositions of the invention will have a low temperature impact strength in excess of about 10.0 ft.lb/inch at -29.degree. C. as measured according to ASTM D256.